The present invention relates to image display devices and more particularly to such devices having a unique electron beam guide allowing proximity focusing of the electron beam.
Recently, several different types of large area image display devices have been suggested utilizing an envelope between 2.5 and 7.6 cm thick with a screen size of approximately 76.times.102 cm. One type of these display devices has a plurality of electron beam guides within the envelope to guide electron beams to various positions on a cathodoluminescent screen as disclosed in copending U.S. patent application Ser. No. 671,358 entitled "Flat Panel Display with Beam Guide" filed on Mar. 29, 1976 by W. W. Siekanowicz et al. These devices are applicable to "element scale displays" where a separate beam guide is employed to direct a beam to each picture element along one horizontal line.
The beam guides comprise electrodes similar to those in FIGS. 12-14 of the above Siekanowicz et al. application which establish symmetrical electric fields to confine and guide the beam in a path along the guide. The guides must be properly positioned within the device so that the high voltage on the screen does not unbalance the symmetrical fields in the guides. Therefore, the screen must be spaced a relatively large distance (e.g. 25 mm) from the guide so that the field strength (volts/mm) on the screen side of the guide is the same as the field strength on the opposite side of the guide. This relatively large distance causes excessive spreading of the beam which reduces image resolution and may adversely affect color purity. To prevent excessive angular beam spreading at the screen, focusing electrodes may be incorporated between the guide and the screen. These additional electrodes complicate manufacturing processes and require additional power supply bias voltages.